Embriology

Department Of General Histology

IntroductionThe Embriology is science, which studies

lows of formation of an embryos and process of his development.

The individual development of living organisms is an an ontogenesis.

Spermatogenesis

Spermatogenesis

Spermatogenesis begins at puberty with a primitive germ cell, the spermatogonium (Gr. sperma + gone, generation), a relatively small round cell, about 12 m in diameter. These cells are located basally in the epithelium next to the basement membrane (Figures 21–5 and 21–6) and different stages of their development are recognized mainly by the shape and staining properties of their nuclei. Spermatogonia with dark, ovoid nuclei act as stem cells, dividing infrequently and giving rise both to new stem cells and to cells with more pale-staining, ovoid nuclei that divide more rapidly as transit amplifying (progenitor) cells (Figure 21–7). These type A spermatogonia each undergo several unique clonal divisions, remaining interconnected as a syncytium (see below), and form type B spermatogonia, which have more spherical pale nuclei.

Testes and seminiferous tubules.

The anatomy of a testis is shown. (a): The diagram shows a partially cut-away sagittal section. (b): The micrograph shows a cross section of one seminiferous tubule.

Spermiogenesis

The diagram depicts the major morphological changes that occur within spermatids as they undergo the differentiation process, called spermiogenesis, and become highly specialized sperm cells. These changes involve flattening of the nucleus, formation of an acrosome which resembles a large lysosome, growth of a flagellum (tail) from the basal body, reorganization of the mitochondria in the midpiece region, and shedding of unneeded cytoplasm as a residual body.

Seminiferous tubules: Sertoli cells and spermatogenesis

In the two cross-sections of seminiferous tubules shown, most of the associated cell types can be seen. Outside the tubules are myoid cells (M) and fibroblasts (F). Inside near the basement membrane are many prominent spermatogonia (SG), small cells which divide mitotically but give rise to a population that enters meiosis. The meiotic cells grow and undergo chromosomal synapsis to become primary spermatocytes (PS), arrested for 3 weeks in prophase of the first meiotic division during which recombination occurs. Primary spermatocytes are the largest spermatogenic cells and are usually abundant at all levels between the basement membrane and the lumen. Each divides to form two secondary spermatocytes, which are seldom seen in sections because they undergo the second meiotic division almost immediately to form two haploid spermatids. Newly formed round spermatids (RS) differentiate and lose volume in becoming late spermatids (LS) and finally motile, highly specialized sperm cells. All stages of spermatogenesis and spermiogenesis occur with the cells intimately associated with the surfaces of adjacent Sertoli cells (SC) which perform several supportive functions.

Clonal nature of spermatogenesis

The diagram shows the clonal nature of the germ cells during spermatogenesis. A subpopulation of type A spermatogonia act as stem cells, dividing to produce new stem cells and other type A spermatogonia that undergo transit amplification as progenitor cells for spermatocytes. Mitosis in these cells occurs with incomplete cytokinesis, leaving the cytoplasm of most or all of these cells connected by intercellular bridges. Type A spermatogonia divide mitotically two or three more times, then differentiate as type B spermatogonia which undergo a final round of mitosis to form the cells that then enter meiosis and become a primary spermatocytes (two are shown), with their cytoplasm still interconnected. The intercellular bridges persist during the first and second meiotic divisions and are finally lost as the haploid spermatids complete their differentiation into sperm (spermiogenesis). During differentiation each spermatid sheds excess cytoplasm as a residual body which is phagocytosed by Sertoli cells, and any germ cells that cannot complete this process and degenerate. The interconnected state of these spermatogonia and the sperm to which they give rise allows free intercellular communication and facilitates their coordinated progress through meiosis and spermiogenesis.

Spermatid in acrosome phase of differentiation A TEM of a spermatid during

the acrosome phase of spermiogenesis shows the nucleus (N) in the center of the cell, half covered by the thin Golgi-derived acrosome (A). The flagellum (F) can be seen emerging from a basal body near the nucleus on the side opposite the acrosome. A cylindrical bundle of microtubules and actin filaments called the manchette (M), surrounds the nucleus behind the acrosome. The manchette is a temporary structure in which vesicles, mitochondria and keratins are shuttled into position as the spermatid elongates in preparation for its final maturation. The spermatid is almost completely surrounded by a Sertoli cell.

Ovums and ovocytes

The ovary produces both oocytes and sex hormones. A diagram of a sectioned ovary (a), shows the different stages of follicle maturation, ovulation, and corpus luteum formation and degeneration. All of the stages and structures shown in this diagram actually would appear at different times during the ovarian cycle and do not occur simultaneously. Follicles are arranged here for easy comparisons. The primordial follicles shown are greatly enlarged. The histological sections identify primordial follicles (b), a primary follicle (c), a secondary follicle (d), and a large vesicular follicle (e). After ovulation, the portion of the follicle left behind forms the corpus luteum (f), which then degenerates into the corpus albicans (g).

Stages of ovarian follicles, from primordial to mature

Diagrams of sectioned ovarian follicles show the changing size and morphology of follicular/granulosa cells at each stage and the disposition of the surrounding thecal cells. However, the relative proportions of the follicles are not maintained in the series of drawings: mature follicles are much larger relative to the early follicles.

Primordial ovarian follicles

The cortical region of an ovary is surrounded by the surface epithelium (SE), a mesothelium with usually cuboidal cells. This layer is sometimes called the germinal epithelium because of an early erroneous view that it was the source of oogonia precursor cells. Underlying the epithelium is a connective tissue layer, the tunica albuginea (TA). Groups of primordial follicles, each formed by an oocyte (O) surrounded by a layer of flat epithelial follicular cells (arrows), are present in the ovarian connective tissue (stroma).

Ovulation At ovulation the large mature primary oocyte escapes from the ovary and is

caught by the dilated end of the uterine tube which is closely applied to the ovarian surface at that time. Ovulation normally occurs midway through the menstrual cycle, ie, around the fourteenth day of a typical 28-day cycle. In humans usually only one oocyte is liberated during each cycle, but sometimes either no oocyte or two or more simultaneous oocytes may be expelled.

In the hours before ovulation the large mature follicle bulging against the tunica albuginea develops a whitish or translucent ischemic area, the stigma, in which the compaction of the tissue has blocked blood flow. Concurrently the granulosa cells and theca interna begin to secret progesterone as well as estrogen. The stimulus for ovulation is a surge of LH secreted by the anterior pituitary gland in response to the rapidly rising level of estrogen produced by the mature dominant follicle. LH stimulates hyaluronate and prostaglandin synthesis and overall fluid production within the preovulatory follicle. Progesterone, LH and FSH activate several proteolytic enzymes, including plasmin and collagenases, within and around the mature follicle which rapidly weaken the granulosa layer (and the cumulus oophorus) as well as the overlying tunica albuginea. The increasing pressure of the follicular fluid and weakening of the follicular wall lead to ballooning and then rupture of the ovarian surface at the stigma. The oocyte and corona radiata, along with follicular fluid and cells from the cumulus, are expelled through this opening by contraction of theca externa smooth muscle triggered by prostaglandins from the follicular fluid.

Just before ovulation the oocyte completes the first meiotic division, which it began and arrested in prophase during fetal life. The chromosomes are equally divided between the two daughter cells, but one of these retains almost all of the cytoplasm. That cell is now the secondary oocyte and the other becomes the first polar body, a very small nonviable cell containing a nucleus and a minimal amount of cytoplasm. Immediately after expulsion of the first polar body, the nucleus of the oocyte begins the second meiotic division, which arrests this time in metaphase.

The ovulated secondary oocyte adheres loosely to the ovary surface because of the hyaluronate-rich, coagulating follicular fluid released with it and, as described later, is drawn into the opening of the uterine tube where fertilization may occur. If not fertilized within about 24 hours, the secondary oocyte begins to degenerate.

Embryo implantation

The coordination between ovulation and endometrial development results in the embryo arriving as a blastocyst about 5 days after ovulation or fertilization, when the uterus is in the late secretory phase and best prepared for implantation. After the zona pellucida is shed, receptor proteins on embryonic trophoblast cells bind ligands and proteoglycans on the endometrial epithelial cells. The trophoblast sends processes between the latter cells and promotes their apoptotic destruction. The trophoblast now also forms an invasive, outer syncytial layer called the syncytiotrophoblast. MMPs are activated and/or released locally to digest the basal lamina and other stroma components, allowing the developing embryo to become enclosed within the stroma. Until chorionic villi of the early placenta are formed, the implanted embryo absorbs nutrients and oxygen from the local endometrial tissue and lacunae of blood.

Decidua, early placenta, and extraembryonic membranes

Term placenta

At low magnification, a full-term placenta includes sections of many villus stems, containing arteries (A) and (V) of the extraembryonic vasculature, and hundreds of smaller villus branches (arrows) which contain connective tissue and microvasculature. Maternal blood (MB) normally fills the space around all the villi.

At higher magnification, the villus connective tissue (CT) can be seen to still resemble mesenchyme and to be surrounded by epithelial cells of the trophoblast, including both the inner cytotrophoblast epithelium and the overlying syncytial trophoblast. In many areas nuclei of the syncytiotrophoblast layer have formed clusters or knots (K) on the surfaces of villi. The trophoblast separates the sinusoids (S) and other vessels containing fetal blood from the maternal blood (MB) in the intervillus space.

Still higher magnification of the same section shows that the villus branches each contain several capillaries (C) and wide sinusoids (S) filled with fetal blood. By the end of pregnancy cells of the cytotrophoblast have greatly decreased in number in many areas of the villi and only a thin syncytiotrophoblast underlain by basement membrane surrounds the villus in these regions (arrows). The external syncytiotrophoblast surface is densely covered with microvilli which increase the absorptive surface and have many receptors and transporters for uptake of material from maternal blood. The extraembryonic blood vessels become closely associated with these areas of thin trophoblast for maximal diffusion of material between the two pools of blood.

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