The document summarizes several aspects of human reproduction and development: 1) Spermatogenesis begins with spermatogonia stem cells in the testes that undergo mitosis and meiosis to form spermatids, which then differentiate into sperm cells through spermiogenesis. 2) In the ovaries, primordial follicles contain oocytes that mature through a series of stages until ovulation releases a secondary oocyte, which may be fertilized to form an embryo. 3) If fertilization is successful, the embryo implants in the uterine lining and the placenta develops to support fetal growth and development until birth.

1. Department Of General Histology

2. Introduction
 The 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.

3. Spermatogenesis

4. 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.

5. Medical Lecture Notes – All Subjects
USMLE Exam (America) – Practice

7. 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.

8. 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.

9. Seminiferous tubules: Sertoli cells
and spermatogenesis

10.  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.

11. Clonal nature of spermatogenesis

12.  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.

13. 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.

14. Ovums and ovocytes

16.  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).

17. 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.

18. 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).

19. 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.

20.  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.

21. Embryo implantation

22.  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.

23. Decidua, early placenta, and
extraembryonic membranes

24. 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.

25. 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.

26. 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.

27. Thank you for attention!