2.5: Secondary tissues - Biology

2.5: Secondary tissues - Biology

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Secondary tissues are produced in woody plants. The secondary phloem replaces the primary phloem.

Similarly, as the trunk of a woody plant gets larger, the dermal tissue need to be expanded and replaced. New dermal tissue is produced by the cork cambium, which lies beneath the bark.

Cell Adhesion

Rodger P. McEver , Francis W. Luscinskas , in Hematology (Seventh Edition) , 2018

Ligand Binding Versus Cell Adhesion

As with all noncovalent macromolecular interactions, adhesion molecules bind to each other with equilibrium affinities that are defined by their association and dissociation rates. However, the efficiency of cell adhesion is not simply a function of the solution-phase equilibrium affinities of adhesion molecules for one another. Adhesion molecules in cell membranes and matrix are limited primarily to two dimensions, and even low-affinity molecular interactions may stabilize adhesion if there is time for sufficient bonds to form along the plane of cell contact. The efficiency of cell attachment and the ensuing strength of adhesion reflect multiple factors that dictate the probability of formation of bonds between adhesion molecules on cell or matrix surfaces. The kinetics of bond formation and dissociation are especially important for certain kinds of cell adhesion. Furthermore, interactions between cell adhesion molecules are subjected to force, which affects the lifetimes of adhesive bonds. This is particularly true in the circulation, where platelets and leukocytes must rapidly adhere to the blood vessel wall and withstand forces applied by the wall shear stresses of flowing blood. Other factors that affect bond formation include the number of adhesion molecules on a cell or matrix surface, the distance the binding domain of an adhesion receptor protrudes from the cell membrane, the lateral mobility of receptors, receptor dimerization, and the clustering of receptors on microvilli or other membrane domains. Cell adhesion can be further stabilized by events that occur after the initial interactions of adhesion molecules. For example, the cytoplasmic domains of many adhesion molecules bind to cytoskeletal components, allowing clustering of receptors into surface patches that strengthen adhesion, thereby promoting cell spreading or migration.

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Gametogenesis (Spermatogenesis and Oogenesis)

Spermatogenesis and oogenesis are both forms of gametogenesis, in which a diploid gamete cell produces haploid sperm and egg cells, respectively.

Learning Objectives

Distinguish between spermatogenesis and oogenesis

Key Takeaways

Key Points

  • Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.
  • In oogenesis, diploid oogonium go through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest it will finish this division as it develops in the follicle, giving rise to a haploid secondary oocyte and a smaller polar body.
  • The secondary oocyte begins the second meiotic division and then arrests again it will not finish this division unless it is fertilized by a sperm if this occurs, a mature ovum and another polar body is produced.
  • In spermatogenesis, diploid spermatogonia go through mitosis until they begin to develop into gametes eventually, one develops into a primary spermatocyte that will go through the first meiotic division to form two haploid secondary spermatocytes.
  • The secondary spermatocytes will go through a second meiotic division to each produce two spermatids these cells will eventually develop flagella and become mature sperm.

Key Terms

  • spermatocyte: a male gametocyte, from which a spermatozoon develops
  • oocyte: a cell that develops into an egg or ovum a female gametocyte
  • polar body: one of the small cells that are by-products of the meiosis that forms an egg
  • mitosis: the division of a cell nucleus in which the genome is copied and separated into two identical halves. It is normally followed by cell division
  • meiosis: cell division of a diploid cell into four haploid cells, which develop to produce gametes

Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell’s life cycle, resulting in gametes that each contain half the number of chromosomes as the parent. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis.


Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.

Oogenesis: The process of oogenesis occurs in the ovary’s outermost layer. A primary oocyte begins the first meiotic division, but then arrests until later in life when it will finish this division in a developing follicle. This results in a secondary oocyte, which will complete meiosis if it is fertilized.

The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.


Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm.

Spermatogenesis: During spermatogenesis, four sperm result from each primary spermatocyte, which divides into two haploid secondary spermatocytes these cells will go through a second meiotic division to produce four spermatids.

Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. When it reaches the lumen of the tubule and grows a flagellum (or “tail”), it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.

Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.

Genetic Control of Flowers

A variety of genes control flower development, which involves sexual maturation and growth of reproductive organs as shown by the ABC model.

Learning Objectives

Diagram the ABC model of flower development and identify the genes that control that development

Key Takeaways

Key Points

  • Flower development describes the process by which angiosperms (flowering plants) produce a pattern of gene expression in meristems that leads to the appearance of a flower the biological function of a flower is to aid in reproduction.
  • In order for flowering to occur, three developments must take place: (1) the plant must reach sexual maturity, (2) the apical meristem must transform from a vegetative meristem to a floral meristem, and (3) the plant must grow individual flower organs.
  • These developments are initiated using the transmission of a complex signal known as florigen, which involves a variety of genes, including CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T.
  • The last development (the growth of the flower’s individual organs) has been modeled using the ABC model of flower development.
  • Class A genes affect sepals and petals, class B genes affect petals and stamens, class C genes affect stamens and carpels.

Key Terms

  • sepal: a part of an angiosperm, and one of the component parts of the calyx collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
  • stamen: in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
  • verticil: a whorl a group of similar parts such as leaves radiating from a shared axis
  • biennial: a plant that requires two years to complete its life cycle
  • whorl: a circle of three or more leaves, flowers, or other organs, about the same part or joint of a stem
  • apical meristem: the tissue in most plants containing undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place at the tip of a root or shoot.
  • angiosperm: a plant whose ovules are enclosed in an ovary
  • perennial: a plant that is active throughout the year or survives for more than two growing seasons
  • primordium: an aggregation of cells that is the first stage in the development of an organ

Genetic Control of Flowers

Flower development is the process by which angiosperms produce a pattern of gene expression in meristems that leads to the appearance of a flower. A flower (also referred to as a bloom or blossom) is the reproductive structure found in flowering plants. There are three physiological developments that must occur in order for reproduction to take place:

Anatomy of a flower: Mature flowers aid in reproduction for the plant. In order to achieve reproduction, the plant must become sexually mature, the apical meristem must become a floral meristem, and the flower must develop its individual reproductive organs.

  1. the plant must pass from sexual immaturity into a sexually mature state
  2. the apical meristem must transform from a vegetative meristem into a floral meristem or inflorescence
  3. the flowers individual organs must grow (modeled using the ABC model)

Flower Development

A flower develops on a modified shoot or axis from a determinate apical meristem (determinate meaning the axis grows to a set size). The transition to flowering is one of the major phase changes that a plant makes during its life cycle. The transition must take place at a time that is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success. In order to flower at an appropriate time, a plant can interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes. Many perennial and most biennial plants require vernalization to flower.

Genetic Control of Flower Development

When plants recognize an opportunity to flower, signals are transmitted through florigen, which involves a variety of genes, including CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T. Florigen is produced in the leaves in reproductively favorable conditions and acts in buds and growing tips to induce a number of different physiological and morphological changes.

From a genetic perspective, two phenotypic changes that control vegetative and floral growth are programmed in the plant. The first genetic change involves the switch from the vegetative to the floral state. If this genetic change is not functioning properly, then flowering will not occur. The second genetic event follows the commitment of the plant to form flowers. The sequential development of plant organs suggests that a genetic mechanism exists in which a series of genes are sequentially turned on and off. This switching is necessary for each whorl to obtain its final unique identity.

ABC Model of Flower Development

In the simple ABC model of floral development, three gene activities (termed A, B, and C-functions) interact to determine the developmental identities of the organ primordia (singular: primordium) within the floral meristem. The ABC model of flower development was first developed to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids and the Asterids both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each verticil.

In the first floral whorl only A-genes are expressed, leading to the formation of sepals. In the second whorl both A- and B-genes are expressed, leading to the formation of petals. In the third whorl, B and C genes interact to form stamens and in the center of the flower C-genes alone give rise to carpels. For example, when there is a loss of B-gene function, mutant flowers are produced with sepals in the first whorl as usual, but also in the second whorl instead of the normal petal formation. In the third whorl the lack of B function but presence of C-function mimics the fourth whorl, leading to the formation of carpels also in the third whorl.

ABC model of flower development: Class A genes (blue) affect sepals and petals, class B genes (yellow) affect petals and stamens, class C genes (red) affect stamens and carpels.

Most genes central in this model belong to the MADS-box genes and are transcription factors that regulate the expression of the genes specific for each floral organ.

Types of Meristematic Tissue

There are three types of meristematic tissues, categorized according to where they appear in the plant: "apical" (at the tips), "intercalary" (at the middle), and "lateral" (at the sides).

The apical meristematic tissues are also known as "primary meristematic tissues," because these are what form the main body of the plant, allowing for vertical growth of stems, shoots, and roots. The primary meristem is what sends a plant's shoots reaching for the sky and the roots burrowing into the soil.

Lateral meristems are known as "secondary meristematic tissues" because they are what is responsible for an increase in girth. The secondary meristematic tissue is what increases the diameter of tree trunks and branches, as well as the tissue that forms bark.

Intercalary meristems occur only in plants that are monocots, a group that includes the grasses and bamboos. Intercalary tissues located at the nodes of these plants allow the stems to regrow. It is intercalary tissue that causes grass leaves to grow back so quickly after being mowed or grazed.

Physiotherapy Management [ edit | edit source ]

This classification is based on a treatment protocol of Clanton et al. [8] , but it is similar to other classifications. It is possible that some phases overlap, dependable on the individual response to healing and the type of injury. Not every patient undergoes all phases to achieve full rehabilitation.

Phase 1: Acute Phase (1 - 7 Days) [ edit | edit source ]

  • Goal: Minimize inflammation and pain.
  • Treatment:
      -method: Rest, ice, compression and elevation
  • Pain-free range of motion with cryotherapy
  • Phase 2: Subacute Phase (Day 3 - < 3 Weeks) [ edit | edit source ]

    This phase starts when signs of inflammation begin to reduce. Inflammation signs are heat, swelling, redness and pain.

    • Goal: Prevent muscle atrophy
    • Treatment:
      • Pain-free full range of motion: concentric strengthening
      • If any pain present: decrease the intensity of exercises

      Phase 3: Remodelling Phase: ( 1 - 6 Weeks) [ edit | edit source ]

      • Stretching to avoid a decrease in flexibility
      • Eccentric strengthening
      • It is important to make sure that the muscle is already regenerated, to prevent risk of re-injury

      Phase 4: Functional Phase: (2 Weeks - 6 Months) [ edit | edit source ]

      • Goal: Return to sport without re-injury.
      • Treatment:
        • Increase their strength, endurance, speed, agility, flexibility and proprioception
        • Sport-specific activities

        Phase 5: Return to Competition Phase: (3 Weeks to 6 Months) [ edit | edit source ]

        • Goal: Avoid a re-injury
        • Criteria: Full range of motion, strength, coordination and psychological readiness
        • Treatment:
          • Address deficits in criteria
          • Progressive agility and trunk stabilization [9]

          Difference Between Primary and Secondary Lymphoid Organs

          The human immune system is an important system, which facilitates major defensive actions against the foreign particles and microorganisms. The tissue aggregates where leukocytes mature, differentiate, and proliferate are called the lymphoid organs. They are mainly composed of epithelial cells and stromal cells, arranged either into organs or accumulation of diffuse lymphoid tissues. Lymphoid organs are classified as primary and secondary lymphoid organs.

          Primary Lymphoid Organs

          Primary lymphoid organs include the thymus and bone marrow. They are the places where the B and T lymphocytes differentiate from stem cells therefore, called as the sites of lymphopoiesis. These organs were first discovered in birds, in which the maturation of B lymphocyte takes place in the bursa of Fabricius. Humans do not possess this organ. In humans, B lymphocytes mature and differentiate from hematopoietic stem cells in the fetal liver during the embryonic life. After birth, the maturation and differentiation of B cells take place in the bone marrow. Progenitor cells of bone marrow differentiate into T lymphocytes, once they migrate to the thymus. Thus, the major function of the thymus is to direct T lymphocytes to differentiate between self and nonself antigens.

          Secondary Lymphoid Organs

          Secondary lymphoid organs include the lymphoid nodes, Peyer’s patches, spleen, tonsils and adenoids. They are the sites where the antigen- driven proliferation and differentiation, and lymphocyte respond to pathogens and foreign antigens take place. Infectious organisms are likely to be found in these organs.

          What is the difference between Primary and Secondary Lymphoid Organs?

          • Primary lymphoid organs develop before secondary organs during the ontogeny.

          • Primary lymphoid organs are the thymus and bone marrow, whereas secondary lymphoid organs are the lymphoid nodes, Peyer’s patches, tonsils, adenoids and spleen.

          • Primary lymphoid organs are the site of maturation for T and B cells, whereas secondary lymphoid organs are the sites of cell function for mature T and B cells.

          • Differentiation of lymphocytes is taken place in primary lymphoid organs while the interaction of immune cells with each other and antigen processing are taken place in secondary lymphoid organs.

          • Primary lymphoid organs serve as the microenvironment for antigen- independent differentiation of lymphocytes, whereas secondary lymphoid organs serve as the microenvironment for attracting antigen- specific lymphocytes, facilitating the lymphocyte differentiation and distributing the differentiated effector cells or their products to other parts of the body.


          We thank all the members of the Novoa lab for their valuable insights and discussion. The results shown here are in whole or part based upon data generated by the TCGA Research Network. We thank Anaiis Zaratzian for the initial setup of the immunohistochemistry experiments in the Histopathology Facility at the Garvan Institute. We also thank IRB Histopathology Facility and CRG Histology Unit for the immunohistochemistry experiments and tissue sectioning,

          Peer review information

          Barbara Cheifet was the primary editor of this article and managed its editorial process and peer review in collaboration with the rest of the editorial team.

          Review history

          The review history is available as Additional file 14.


          OB is supported by a UNSW International PhD fellowship. MCL is supported by an FPI Severo Ochoa PhD fellowship from the Spanish Ministry of Economy, Industry and Competitiveness (MEIC). EMN was supported by a Discovery Early Career Researcher Award (DE170100506) from the Australian Research Council and is currently supported by CRG Severo Ochoa Funding. This work was supported by the Australian Research Council (DP180103571 to EMN), by the Spanish Ministry of Economy, Industry and Competitiveness (PGC2018-098152-A-100 to EMN) and NHMRC funds (Project Grant APP1070631 to JSM). We acknowledge the support of the Spanish Ministry of Economy, Industry and Competitiveness (MEIC) to the EMBL partnership, Centro de Excelencia Severo Ochoa, and CERCA Programme / Generalitat de Catalunya.

          Availability of data and materials

          All scripts used in this work have been made publicly available and can be found at [101]. All datasets used to build the figures, as well as intermediate analysis files (alignment files, maximum likelihood trees, scatter plots of tissue specificity, barplots of amniote and primate ortholog expressions, scatter plots of tumor vs normal tissues, boxplots of individual expression of RMPs in tumor-normal paired tissues and survival plots), are publicly available at Raw immunofluorescence images and IHC scans have been deposited in Figshare [102, 103].

          Third-party mRNA expression data used throughout this work were obtained from the following resources: (i) mRNA expression datasets across human tissues were obtained from GTEx ( [45] and HPA ( [96] (ii) mRNA expression datasets for mouse tissues were obtained from ENCODE ( [97] (iii) mRNA expression levels across tissues from 12 amniote species were obtained from GSE30352 [99] (iv) single-cell RNASeq levels during mouse spermatogenesis was obtained from GSE112393 [55], GSE125372 [58, 59], and GSE113293 [59] (v) mRNA expression data from tumor-normal human samples were downloaded from the UCSC XENA Project ( [73] (vi) survival phenotypes were downloaded from the XENA Platform (, using the “TCGA TARGET GTEX” cohort [73].

          Section Summary

          The reproductive structures that evolved in land animals allow males and females to mate, fertilize internally, and support the growth and development of offspring. Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.

          The male and female reproductive cycles are controlled by hormones released from the hypothalamus and anterior pituitary and hormones from reproductive tissues and organs. The hypothalamus monitors the need for FSH and LH production and release from the anterior pituitary. FSH and LH affect reproductive structures to cause the formation of sperm and the preparation of eggs for release and possible fertilization. In the male, FSH and LH stimulate Sertoli cells and interstitial cells of Leydig in the testes to facilitate sperm production. The Leydig cells produce testosterone, which also is responsible for the secondary sexual characteristics of males. In females, FSH and LH cause estrogen and progesterone to be produced. They regulate the female reproductive cycle, which is divided into the ovarian cycle and the menstrual cycle.

          Human pregnancy begins with fertilization of an egg and proceeds through the three trimesters of gestation. The first trimester lays down the basic structures of the body, including the limb buds, heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems. The third trimester exhibits the greatest growth of the fetus and culminates in labor and delivery. The labor process has three stages (contractions, delivery of the fetus, and expulsion of the placenta), each propelled by hormones.


          1. Which of the following statements about the male reproductive system is false?
            1. The vas deferens carries sperm from the testes to the seminal vesicles.
            2. The ejaculatory duct joins the urethra.
            3. Both the prostate and the bulbourethral glands produce components of the semen.
            4. The prostate gland is located in the testes.
            1. LH and FSH are produced in the pituitary, and estrogen and progesterone are produced in the ovaries.
            2. Estradiol and progesterone secreted from the corpus luteum cause the endometrium to thicken.
            3. Both progesterone and estrogen are produced by the follicles.
            4. Secretion of GnRH by the hypothalamus is inhibited by low levels of estrogen but stimulated by high levels of estrogen.
            1. scrotum
            2. seminal vesicles
            3. seminiferous tubules
            4. prostate gland
            1. labia minora
            2. breast
            3. ovaries
            4. uterus
            1. testosterone
            2. estrogen
            3. GnRH
            4. progesterone
            1. the placenta
            2. diffusion through the endometrium
            3. the chorion
            4. the blastocyst
            1. oxytocin
            2. estrogen
            3. β-HCG
            4. progesterone
            1. D
            2. C
            3. C
            4. D
            5. C
            6. B
            7. A
            8. Stem cells are laid down in the male during gestation and lie dormant until adolescence. Stem cells in the female increase to one to two million and enter the first meiotic division and are arrested in prophase. At adolescence, spermatogenesis begins and continues until death, producing the maximum number of sperm with each meiotic division. Oogenesis continues again at adolescence in batches of eggs with each menstrual cycle. These primary oocytes finish the first meiotic division, producing a viable egg with most of the cytoplasm and its contents, and a second cell called a polar body containing 23 chromosomes. The second meiotic division is initiated and arrested in metaphase. At ovulation, one egg is released. If this egg is fertilized, it finishes the second meiotic division. This is a diploid, fertilized egg.
            9. Low levels of progesterone allow the hypothalamus to send GnRH to the anterior pituitary and cause the release of FSH and LH. FSH stimulates follicles on the ovary to grow and prepare the eggs for ovulation. As the follicles increase in size, they begin to release estrogen and a low level of progesterone into the blood. The level of estrogen rises to a peak, causing a spike in the concentration of LH. This causes the most mature follicle to rupture and ovulation occurs.
            10. Stage one of labor results in uterine contractions, which thin the cervix and dilate the cervical opening. Stage two delivers the baby, and stage three delivers the placenta.


            bulbourethral gland: the paired glands in the human male that produce a secretion that cleanses the urethra prior to ejaculation

            corpus luteum: the endocrine tissue that develops from an ovarian follicle after ovulation secretes progesterone and estrogen during pregnancy

            clitoris: a sensory and erectile structure in female mammals, homologous to the male penis, stimulated during sexual arousal

            estrogen: a reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium absorption

            follicle stimulating hormone (FSH): a reproductive hormone that causes sperm production in men and follicle development in women

            gestation: the development before birth of a viviparous animal

            gestation period: the length of time of development, from conception to birth, of the young of a viviparous animal

            gonadotropin-releasing hormone (GnRH): a hormone from the hypothalamus that causes the release of FSH and LH from the anterior pituitary

            human beta chorionic gonadotropin (β-HCG): a hormone produced by the chorion of the zygote that helps to maintain the corpus luteum and elevated levels of progesterone

            inhibin: a hormone made by Sertoli cells, provides negative feedback to hypothalamus in control of FSH and GnRH release

            interstitial cell of Leydig: a cell type found next to the seminiferous tubules that makes testosterone

            labia majora: the large folds of tissue covering inguinal area

            labia minora: the smaller folds of tissue within labia majora

            luteinizing hormone (LH): a reproductive hormone in both men and women, causes testosterone production in men and ovulation and lactation in women

            menstrual cycle: the cycle of the degradation and re-growth of the endometrium

            oogenesis: the process of producing haploid eggs

            ovarian cycle: the cycle of preparation of egg for ovulation and the conversion of the follicle to the corpus luteum

            oviduct: (also, fallopian tube) the muscular tube connecting uterus with ovary area

            ovulation: the release of an oocyte from a mature follicle in the ovary of a vertebrate

            penis: the male reproductive structure for urine elimination and copulation

            placenta: the organ that supports the transport of nutrients and waste between the mothers and fetus’ blood in eutherian mammals

            progesterone: a reproductive hormone in women assists in endometrial regrowth and inhibition of FSH and LH release

            prostate gland: a structure that is a mixture of smooth muscle and glandular material and that contributes to semen

            scrotum: a sac containing testes, exterior to body

            semen: a fluid mixture of sperm and supporting materials

            seminal vesicle: a secretory accessory gland in male contributes to semen

            seminiferous tubule: the structures within which sperm production occurs in the testes

            Sertoli cell: a cell in the walls of the seminiferous tubules that assists developing sperm and secretes inhibin

            spermatogenesis: the process of producing haploid sperm

            testes: a pair of male reproductive organs

            testosterone: a reproductive hormone in men that assists in sperm production and promoting secondary sexual characteristics

            uterus: a female reproductive structure in which an embryo develops

            vagina: a muscular tube for the passage of menstrual flow, copulation, and birth of offspring

            Watch the video: ΒΙΟΛΟΓΙΑ Β ΛΥΚΕΙΟΥ, 1ο Κεφάλαιο - Ενότητα: Πρωτεΐνες, Αμινοξέα u0026 Επίπεδα οργάνωσης των πρωτεϊνών (February 2023).